Ion sources of this type are already known which comprise, in vacuo, a source of neutral particles of the same nature as the ions to be produced, an ionization support which possesses at least one active surface suitable for adsorbing the neutral particles, and then desorbing them as ions, means for bringing the neutral particles to the ionization support which then transforms them into ions by adsorption/desorption, and means for directing the major portion of the ions produced in this way into a beam which is emitted in a predetermined direction.
It has been known for a long time that an atom may be desorbed from a hot surface as a positive or negative ion. The main parameters governing this phenomenon are firstly the temperature of the ionization support and the work function, i.e. the work required to extract an electron, and secondly the ionizing propensity of the desorbing element. This propensity is expressed by the ionization potential or by the electron affinity, depending on whether the ionization is positive or negative.
The degree of ionization obtained during such desorption is governed by the Saha-Langmuir equation. This equation shows that the degree of ionization is an exponential function of the difference between the work required to leave the heated support and either the ionization potential for positive ions or the electron affinity for positive ions.
In order to produce given ions, it is possible to obtain an ionization probability close to unity by choosing a suitable material for the ionization support. Support temperature then has only a small effect on ionization probability. However, temperature is the key factor governing the desorption process. In particular, temperature affects the length of time which an adsorbed atom remains on the surface of the support.
Thus, the degree to which a hot surface receiving, for example, a jet of alkali atoms such as potassium, rubidium, or cesium, is covered in adsorbed atoms per unit area under steady state conditions without adsorbed ions accumulating, depends on the incident flux of neutral atoms and on the temperature of the support. However, the presence of the adsorbed atoms modifies the work functions and can thus have an effect on ionization probability, and in particular may considerably reduce it. It thus appears that ion sources operate in a complex manner.
One of the main qualities of an ion source is its brightness which may be defined by the expression: EQU dI=B.ds.d.OMEGA..dE
where dI is the intensity of the beam emitted by a surface element ds in a solid angle d.OMEGA. defined about a direction which is defined by angles .theta. and .phi., and in an energy band lying between E and E+dE. The brightness B is a function of .theta., .phi. and E.
As a simplified example, consider a plane emitting surface which is parallel to a plane electrode having a round hole therethrough. A positive or negative voltage V is established between the emitting surface and the electrode which is placed at ground potential. Assume that B is independent of the azimuth angle .phi. and that it varies as a function of .theta., the angle of the emission direction to the normal, in accordance with Lambert's cosine law. The brightness B can then be written: EQU B=V/.pi.E.sub.o.dJ.sub.o /dE.sub.o
where E.sub.0 is the initial energy of a particle leaving the surface nd J.sub.0 is the current density (current per unit area) of the particles at the emitting surface. In may be observed in this example that thermal ion sources provide a low value for the initial energy E.sub.0 of an ion leaving the surface. It may also be observed that the arrival function which defines the incident flux of neutral particles has a major effect since this function controls the current density J.sub.0.
Ion sources are already known in which the ionizing member is a pellet of sintered tungsten. An alkali vapor passes into the interstices which remain between the tungsten grains. The pellet is raised to a temperature of about 1200.degree. C., and it is placed in an electric field for accelerating the ions which emerge from between the grains. The source of neutral particles is a pool of liquid cesium whose temperature is adjusted to obtain a cesium vapor pressure which is high enough to cause said vapor to diffuse through the pores of the sintered tungsten pellet. This first known source of ions thus has the peculiarity of the atoms to be ionized passing through the ionization support.
This type of ion source is capable of delivering high currents, so long as a large emitting surface area is used.
This is a considerable handicap when an ion sonde or probe is to be produced, i.e. a source of ions which produces a narrow beam. A priori it is difficult to make such an emitting surface small, and it is therefore necessary to work with a relatively large emitting surface with a major portion of the ions produced therefrom being subsequently eliminated by diaphragms.
Ion sources are also known which use a hot filament. They are assembled in substantially the same manner as an electron gun: a hair-pin shaped filament is placed in the middle of a circular orifice through an electrode which acts both as a screen grid and as a control grid. The filament and the control grid are both raised to a high positive voltage and are disposed opposite an electrode at ground potential having a circular hole therethrough (equivalent to the anode of an electron gun). The space between the filament and this "anode" is filled with cesium vapor from an adjoining oven. Cesium atoms which are ionized on the tip of the filament are accelerated by the electric field and leave via the hole through the "anode" and they appear to come from a virtual source of small size. This known assembly provides a sufficiently small source to make an ion probe. However, it suffers from two major drawbacks: the first is that above 5 kilovolts (kV), flash-overs become frequent for reasons which are difficult to master such as the insulators becoming coated in metal and parasitic emission of electrons; and the second is that the cesium vapor escapes through the outlet hole and condenses in other parts of the installation.
Ion sources are also known in which the ionization support is in the form of a baffle or chicane, thereby reducing the number of neutral particles which pass into the emitted ion beam.
Such equipment is described in French certificate of addition No. 65 999 which relates to a discharge tube. The baffle is very simple and works on condition that the neutral atoms propagate in straight lines. However, the ion source in this prior document has low brightness, is subject to high energy dispersion, and is rather large. Further, it appears to suffer from lack of stability and to tolerate the vapor filling the inside of the discharge tube, since this is acceptable in discharge tubes.
Up to a point, U.S. Pat. No. 3,283,193 may also be considered as describing a baffle, in the rather specialized context of catalytically producing nascent hydrogen. Electron bombardment ionizes a portion of the hydrogen atoms before they have the time to recombine into molecules. Here again, it is clear that the ion source made in this way has low brightness, is large, and is dispersed in energy. It is also fairly unstable, and vapor is liberated outside the ionizer itself, since few hydrogen atoms are effectively ionized.
Under such conditions, the present invention provides a novel ion source having manifest advantages over prior art ion sources, and in particular:
a very small emitting surface area, which may be very bright when required;
no direct flux of neutral atoms or particles in the remainder of the installation;
an accelerating voltage greater than 10 kV without causing flash-overs;
it uses a solid source of neutral particles operating at low pressure and thus avoiding the use of liquid metal;
a stable beam of ions with low energy dispersion; and
an ion beam of well-determined geometry, thereby avoiding electrode erosion by cathode sputtering.